A. Technical field of the Invention
This invention relates to an atomic force microscope for observing a surface of a sample with atomic order resolution.
B. Description of the Related Art
An atomic force microscope (AFM) includes a flexible cantilever having a probe on the free end, and is used to observe a surface of a sample with atomic order resolution by detecting variation of the displacement of the free end caused by an atomic force exerted between the probe and the sample, and scanning the probe across the surface of the sample with the variation kept constant.
In such AFM, an optical interference method, an optical focus detecting method and an optical lever method are some examples of a method for detecting displacement of the free end of the cantilever incorporated. In the case of the optical interference method, a laser beam is radiated onto a mirror provided on a surface of the free end, which is opposite to that on which the probe is fixed, and that variation in the intensity of the beam reflected therefrom which is due to displacement of the cantilever is detected by use of an interferometer. In the case of the optical focus detecting method, a detector is used to detect whether a beam projected thereon from the mirror surface of the cantilever via an optical element such as a critical angle prism, a cylindrical lens, or a knife edge is focused. In the case of the optical lever method, that variation in the incident angle of a laser beam with respect to the mirror of the cantilever, which is due to displacement thereof, is detected on the basis of the principle of optical lever. In this case, the variation in the incident angle of the laser beam is amplified and detected on the surface of a beam receiver.
A cantilever displacement detecting system used in the optical lever method will be explained with reference to FIG. 1. As is shown in FIG. 1, a cantilever 112 supported by a housing 110 has a free end with a probe 114 fixed on one surface thereof, and a mirror 116 on the other surface. A sample 118 placed on a sample table 120 is positioned opposed to the probe 114 in the vicinity thereof. The probe 114 is moved in X- and Y-directions, i.e., across the surface of the sample. The housing 110 houses a laser diode 122 for radiating a laser beam onto a mirror 116, and a light receiver 124 for receiving the beam reflected from the mirror 116. The receiver 124 has two light receiving regions 124a and 124b, and is located so that the center of the laser beam from the mirror 116 positions onto a boundary between the light receiving regions 124a and 124b when the cantilever 112 is in a reference position to be assumed at the time of measurement (i.e., in a horizontal position in FIG. 1). The light receiving regions 124a and 124b output voltage signals corresponding to the intensities of received light beam components, the inclination of the mirror 116 or the displacement of the cantilever 112 is obtained on the basis of measuring the difference between the intensities.
The detection sensitivity S in the optical lever method is given by EQU S=D/.DELTA.=2L/l
where D represents the amount of variation in the intensity of the beam on the light receiving surface of the light receiver 124, l represents the length of the cantilever 112 (generally 100-200 .mu.m), L represents the length of the path of the beam reflected from the mirror 16 (i.e., the distance between the mirror 116 and the light receiver 124), and .DELTA. represents the amount of displacement of the probe 114.
When L=100 mm and l=200 .mu.m, EQU S=200/(200.times.10.sup.-3)=10.sup.3.
Thus, the optical lever method provides a highly sensitive displacement-detecting system with simple structure. The critical angle method provides a displacement detecting system with simple structure, too. In this method, a prism is located so that a laser beam directs onto its prism surface at a critical angle. The laser beam passes through the prism surface or is reflected to the interior of glass in response to a slight change of its incident angle. This means that the transmittance and the reflectance abruptly vary in accordance with variation in deflection angle. FIG. 2 shows a reflectance curve assumed when the refractive index n of glass is 1.5. Now, an observation optical system integrated AFM disclosed in U.S. application Ser. No. 07/511,054, in which the critical angle method is applied to the cantilever displacement system, will be explained with reference to FIG. 3.
A laser beam emitted from a laser diode 87 is collimated by means of a collimating lens 90, and enters into a polarized beam splitter 86. The laser beam reflected from the polarized beam splitter 86 is further reflected from a half mirror 85 and enters into a 1/4 wavelength plate 84.
On the other hand, an illumination beam emitted from a light source 96 of the observation lighting apparatus is collimated by a lens 97 and reflected by a half mirror 92. The reflected illumination beam passes through a filter 91 and the half mirror 85, and enters into the 1/4 wavelength plate 84.
The laser beam and illumination beam which have different principal rays enter into an objective lens 83 through the 1/4 wavelength plate 84. The laser beam is transformed by the plate 84 from a linearly polarized beam to a circularly polarized beam, and is then converged by the objective lens 83 onto a cantilever 22 having a probe on its free end. The illumination beam is converged to a point in the vicinity of the probe to illuminate the overall visual field.
The illumination beam reflected from a sample 26 passes through the objective lens 83, 1/4 wavelength plate 84, half mirror 85, filter 91, and half mirror 92, and is converged by an image forming lens 93 and enters into a prism 94. Part of the beam having entered into the prism 94 is reflected on its boundary face and reaches an ocular lens 95. The other part of the beam passes through the prism 94 and enters into a video camera 27 equipped with a CCD image element, etc., where it is converted to an image signal. This signal is supplied to a video monitor 28 and displayed thereon. The 1/4 wavelength plate 84 is slightly inclined relative to the optical axis so that the reflected illumination beam will not directly enter into the observation optical system. Thus, a clear visual field observation image without flare is obtained.
The laser beam reflected on the upper surface of the cantilever 22 passes through the objective lens 83 and 1/4 wavelength plate 84, reflects from half mirror 85, and is guided to the polarized beam splitter 86. The laser beam having passed through the 1/4 wavelength plate 84 is converted to a linearly polarized beam whose oscillation phase is rotated by 90.degree. relative to that of the beam before entering the same. The laser beam having entered the beam splitter 86 is divided into two beam components, one of which is radiated via a first critical angle prism 88a onto a first light receiving element 89a including two light receiving portions, and the other of which is radiated via a first critical angle prism 88b onto a second light receiving element 89b including two light receiving portions.
In the above structure, the critical angle method is used to detect the position of the cantilever. The principle of the method will be explained with reference to FIG. 4.
To effect this method, a critical angle prism c is located such that a critical angle is formed between a parallel light beam from a lens b and the reflecting surface a of the prism.
When the reflecting surface a is provided at the focus of the objective lens b (as indicated by solid line B), that is, when the beam can be converged on the surface a, the beam reflected therefrom is transformed by the lens b to a parallel beam, which enters into the critical angle prism c. In this case, the overall beam is reflected on the surface a, and the same amount of light is radiated onto each of the photodiodes of a light receiving element d.
On the other hand, when the reflecting surface a is provided at a location nearer to the lens b than the focus thereof (as indicated by broken line C), the beam reflected from the surface a and having passed through the lens b is a divergent beam, which enters into the critical angle prism c. Further, when the reflecting surface a is provided at a location farther from the lens b than the focus thereof (as indicated by broken line A), the beam having passed through the lens b is a converged beam, which enters into the critical angle prism c. In both cases, a non-parallel beam enters into the prism c. Since only a mid portion of the beam enters into the prism at the critical angle, and the incident angle of the part of the beam positioned on one side from the mid portion is smaller than the critical angle, that part of the beam is radiated partially to the outside of the prism c. Moreover, since the incident angle of the part of the beam positioned on the other side from the mid portion is larger than the critical angle, that part of the beam is overall reflected from the prism c. As a result, the amount of light received by one photodiode of the light receiving element d differs from that received by the other photodiode, and a signal indicative of the difference between the light amounts is output from an output terminal f via a differential amplifier e. This means that the position of the reflecting surface a is detected on the basis of the difference in the light amounts of the detection surfaces of the element d.
As described above, since the beam having entered into the critical angle prism at an incident angle smaller than the critical angle is radiated partially to the outside thereof whenever it is reflected from the reflecting surface, the amount of the refractive component of the beam is greatly reduced. Thus, the difference in the amount of light having entered at an angle smaller than the critical angle and that of light having entered at an angle larger than the same is greatly increased. To enhance the accuracy of measurement, the beam preferably reflects several times in the critical angle prism. In this system, the detection light is reflected twice in the prism. As is shown in FIG. 5, the output of a photodiode PD1 of the first light receiving element is input to the inverted input terminal of a comparator 102, while the output of a photodiode PD2 is input to the non-inverted input terminal of the comparator 102. The comparator 102 outputs the difference between the outputs of the photodiodes PD1 and PD2. On the other hand, the output of a photodiode PD3 of the second light receiving element is input to the inverted input terminal of a comparator 104, and the output of a photodiode PD4 is input to the non-inverted input terminal of the comparator 104. The comparator 104 outputs the difference between the outputs of the photodiodes PD3 and PD4. The outputs of the comparators 102 and 104 are added, and the addition value is input to one terminal of a comparator 106, where the addition value is compared with a reference value, and the comparison result is output. Thus, a signal is output from a terminal 108, which indicates the difference between the amounts of light received in two regions which are defined by the center line of the spot of a beam radiated onto the light receiving element, i.e., which indicates the position of the cantilever 22.
In the above system, the sample 26 is placed on an xyz driving apparatus 24 (e.g. a cylindrical actuator), and is raster-scanned in an xy plane at 10-100 cycles. A signal directly obtained during the raster scanning and indicative of the displacement of the cantilever, or a servo signal obtained when servo control is performed in the z-direction so as to keep the displacement of the cantilever constant, is processed in synchronism with the scanning signal so as to obtain an AFM image.
However, since a cantilever displacement detecting system which employs the above-described interference method, optical lever method, or focus detecting method is inevitably large and heavy, the raster scanning between the probe and sample is performed generally by placing on an actuator a sample such as a several mm-square HOPG chip, Si chip, or semiconductor chip, and moving the sample together with the actuator. The range which can be observed by thin scanning is as extremely narrow as 10-20 .mu.m.sup.2.
However, it has been necessary to measure a component such as an 8-inch wafer, for example, in the case of measuring the configurations of the surface at nm-order, or in the case of observing the structure of a semiconductor during process. Enlarging the stage on which a sample is placed and increasing its rigidity so as to satisfy the necessity inevitably increases its weight. This is disadvantageous to perform raster scanning by moving a large sample, since a great force of inertia exerts during moving it together with a heavy stage, and hence control of the movement of the sample and stage is extremely difficult.